An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211.
Physical vapor deposition in a vacuum environment is the principal means of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate dependant vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties.
To overcome the thermal sensitivity of these materials, only small quantities of organic materials have been loaded in sources and they are heated as little as possible. In this manner, the material is consumed before it has reached the temperature exposure threshold to cause significant degradation. The limitations with this practice are that the available vaporization rate is very low due to the limitation on heater temperature, and the operation time of the source is very short due to the small quantity of material present in the source. The low deposition rate and frequent source recharging place substantial limitations on the throughput of OLED manufacturing facilities.
A secondary consequence of heating the entire organic material charge to roughly the same temperature is that it is impractical to mix additional organic materials, such as dopants, with a host material unless the vaporization behavior and vapor pressure of the materials are very close. This is generally not the case, and prior art devices frequently require the use of separate sources to co-deposit host and dopant materials. These multiple sources must be maintained in an angled arrangement so that the evaporated materials from each source converge at a common point on an OLED substrate. This use of multiple spaced-apart sources leads to obvious limitations in the number of materials that can be co-deposited and obvious deficiencies in the homogeneity of the host and dopant films.
The organic materials used in OLED devices have a highly non-linear vaporization-rate dependence on source temperature. A small change in source temperature leads to a very large change in vaporization rate. Despite this, many devices employ source temperature as the only means to control vaporization rate. To achieve good temperature control, prior art deposition sources typically utilize heating structures whose solid volume is much larger than the organic charge volume, composed of high thermal-conductivity materials that are well insulated. The high thermal conductivity insures good temperature uniformity through the structure and the large thermal mass helps to maintain the temperature within a critically small range by reducing temperature fluctuations. These measures have the desired effect on steady-state vaporization rate stability but have a detrimental effect at start-up. It is common that these devices must operate for many hours at start-up before steady state thermal equilibrium and hence a steady vaporization rate is achieved.
A further limitation of such sources is that the geometry of the vapor manifold changes as the organic material charge is consumed. This change requires that the heater temperature change to maintain a constant vaporization rate. It is observed that the overall plume shape of the vapor exiting the orifices can change as a function of the organic material thickness and distribution in the source, particularly when the conductance to vapor flow in the source with a full charge of material is low enough to sustain pressure gradients from non-uniform vaporization within the source. In this case, as the material charge is consumed, the conductance increases and the pressure distribution and hence overall plume shape improve.
Some organic materials are extremely thermally sensitive and have relatively low vapor pressures. Difficulty has been encountered when vaporizing these materials, even where the material is metered from a cold reservoir to a heating region on as needed basis. The powder feeding path in prior art devices allows heat to be conducted away from the immediately vaporizing material and thereby hinders vaporization of these sensitive and low vapor pressure materials at commercially practical rates. Heat conduction along the powder feeding path in prior art devices has also been a source of degradation for a few extremely thermally sensitive materials.